The present invention relates to a new and distinct cultivar of x Heucherella and given the cultivar name of ‘Onyx’. x Heucherella is in the family Saxifragaceae. x Heucherella ‘Onyx’ originated from a controlled cross between Heuchera ‘B110-5’ (a proprietary, unpatented, unreleased Heuchera), as the seed parent, and Tiarella ‘42-2’ (a proprietary, unpatented, unreleased Tiarella), as the pollen parent.
Compared to the seed parent Heuchera ‘B110-5’, the new cultivar has cut leaves rather than shallowly lobed leaves.
Compared to the pollen parent Tiarella ‘42-2’, the new cultivar has glossy black leaves rather than matte green.
Compared to x Heucherella ‘Burnished Bronze’, U.S. Plant Pat. No. 12,159, the new cultivar has thicker leaves that are much darker and cupped on margins, it is more vigorous, and the habit is larger and more mounding.
This new x Heucherella is uniquely characterized by:                1. leaves dark red brown in spring to almost black for the rest of the year,        2. glossy, palmately cut leaves that are sometimes puckered,        3. a medium mounding habit with an excellent crown count, and        4. excellent vigor.        
This new cultivar has been reproduced only by asexual propagation (cuttings and tissue culture using growing points at the crown). Each of the progeny exhibits identical characteristics to the original plant. Asexual propagation by division and tissue culture using standard micropropagation techniques with terminal and lateral shoots, as done in Canby, Oreg., shows that the foregoing characteristics and distinctions come true to form and are established and transmitted through succeeding propagations. The present invention has not been evaluated under all possible environmental conditions. The phenotype may vary with variations in environment without a change in the genotype of the plant.